极化诱导掺杂AlGaN纳米线及其深紫外LED研究

There are growing demands for mercury-free, environmentally friendly, compact deep ultraviolet light emitting diodes (DUV-LEDs) for applications such as water/air purification and disinfection, food sterilization, free-space non–line-of-sight communication, epoxy curing, bio-sensing etc. The AlxGa1-xN (0≤x≤1) alloy has tunable and direct bandgap ranging from 3.4 to 6.1 eV which covers the UV spectral region from 360 to 210 nm. However, to date, the best-attained wall plug efficiency for AlGaN-based planar DUV-LEDs with operational wavelength (250-280) nm is below 5 percent at 20 mA due to low material quality, extremely poor light extract efficiency, and low current injection efficiency owing to the difficulty of p-type doing in Al-rich AlGaN alloy. To overcome those challenges, this project proposes AlGaN nanowires with polarization-induced p-type doping technique to build high-efficient DUV-LEDs. By linearly change the Al composition in the Al-rich AlGaN alloys along the growth direction in the nanowires, the internal polarization electric field could ionize acceptor dopants using the built-in electronic polarization and hence achieving effective p-type doping. Meanwhile, due to the natural existence of light scattering effect among nanowires, more DUV photons (both TE and TM mode) can escape from the active region and hence be extracted from top surface. Lastly, originating from the large surface to bulk volume ratio, the nanowire lateral dimension reduces, the critical thickness approaches infinity, thus, AlGaN nanowires can be formed virtually on any foreign substrates, such as metal, while maintaining superior crystallinity. This immediately enables their unrivaled capability to take advantage of the metal substrates which are not only electrically conductive but more importantly, having superior thermal conductivity to avoid LED efficiency droop at high current injection, compared to the LEDs on sapphire. Our approach represents a viable path toward high-efficient AlGaN-based DUV-LEDs.

宽禁带半导体AlGaN基深紫外光源在水和空气净化、杀菌、环保、医疗、军事保密通讯等领域具有巨大的应用前景。但由于高Al组分AlGaN材料晶体质量差、p型掺杂难度高、器件光提取效率低等难题，传统AlGaN基平面结构深紫外LED的电光转换效率一般不足5%。本项目利用极化诱导掺杂纳米线LED结构，在改善晶体质量、提高p型AlGaN材料导电特性的同时增大器件光提取效率。拟深入研究Al组分渐变梯度对材料内诱导极化电场的影响，进而探索该电场对p型高Al组分AlGaN纳米线空穴浓度的调控规律；控制并调节MBE生长条件，优化纳米线尺寸和分布规律，利用纳米线之间的光散射效应，提高LED光提取效率；最终，基于纳米线独特的不需要衬底晶格匹配生长条件，在金属等导热导电衬底上生长高Al组分AlGaN纳米线LED，缓解功率衰减，制备高发光效率深紫外LED。

紫外光电器件（LED和探测器等）不仅在杀菌消毒、医疗、工业装备等国民经济领域，而且在非视距保密通讯等军事中有着重要的应用前景。氮化镓(GaN)基可见光波段光电器件已经大规模普及和应用，而在GaN中掺入铝(Al)所形成的AlGaN半导体是被公认为制备半导体紫外光电子器件的理想材料。本项目围绕宽禁带AlGaN基半导体材料外延及其在紫外光电器件中的应用基础为核心研究内容，立足于“纳米结构和薄膜结构的材料外延生长和器件结构创新”，围绕器件中的“载流子输运和复合”这两个最核心物理过程，开展了系统深入的研究，取得一系列研究成果：在紫外LED光电性能研究方面，通过设计和采用不同的LED结构，如N极性AlGaN的紫外LED，倒V型量子垒创新结构，铝组分渐变的末端量子垒结构，量子垒中引入了高铝组分的插入层等创新结构，最终提出能带工程的概念以解决深紫外LED电光转换效率低下的问题；此外，通过研制深紫外micro-LED单颗器件，并通过研制阵列的技术路线来达到类似大尺寸深紫外LED的发光强度。研究发现，利用深紫外micro-LED阵列，整体发光功率提升近50%，在相同的注入电流水平下micro-LED发光阵列比常规大尺寸深紫外LED的发光强度高52%，外量子效率高22%，并用于紫外光通信的应用中。在紫外探测器研究方面，我们利用分子束外延技术所制备的高晶体质量GaN纳米线，通过将GaN纳米线中的经典半导体物理过程与化学反应过程相结合，有望突破传统III族氮化物纳米线的功能限制，拓展新的应用场景，因此开展了系列研究工作。在构建高性能光电化学光探测器的基础上, 通过将光电化学光探测器中载流子的产生、分离及传输过程与电子和空穴在半导体表面/电解液界面处的氧化/还原反应过程相结合，实现了载流子输运过程的有效调制，在该器件中观察到独特的双向光电流现象等，并应于与水下紫外光通信的应用之中。在本项目的资助下，团队以通讯作者发表SCI论文19篇，包括1篇Nature Electronics， 1篇Light: Science and Application封面论文，4篇Adv. Funct. Mater等研究论文，共获得授权发明专利5项，取得了较好的研究成果，顺利结题。